Centrifugal compressors generally include a shaft and one or more rotating impellers attached thereto that are configured to increase the pressure of a working fluid. As the working fluid progresses axially along the compressor through the various stages of rotating impellers, its pressure is increased. Increasing the pressure of the working fluid, however, generates a pressure differential across each impeller which applies an axial thrust in the direction opposite the axial progression of the working fluid. To ensure safe compressor operation, this force must be balanced both statically and dynamically. Typically, the axial thrust is counterbalanced by employing one or more thrust bearings, balance pistons, or a combination of both. The impeller thrust not balanced by the balance piston is generally absorbed by the axial thrust bearing.
A thrust bearing assembly usually consists of thrust bearing halves disposed about the side surfaces of a radially-extending axial bearing disk. The axial bearing disk can be machined directly on the shaft or otherwise coupled thereto by interference fit or using appropriate alternate methods. Magnetic thrust bearings, active and/or passive, are increasingly being used in centrifugal compressors. Passive magnetic thrust bearings, however, cannot be adjusted in real-time and therefore oftentimes fail to provide adequate balance during anomalous thrust loads. Active magnetic bearings, on the other hand, require a continuous power supply, an expensive feedback control module for adjusting the corresponding bearing force, and backup bearings for protecting the axial thrust bearing in the event of a power failure.
A balance piston typically includes a disk attached to the shaft behind the last impeller stage. The outboard side of the disk is subjected to a low pressure from the inlet side of the compressor or from an alternate location within an intermediate stage, thereby creating a pressure differential opposite the direction of the axial thrust created by the impellers. This pressure differential causes a force on the balance piston that counteracts some of the axial forces generated by the impellers. The pressure differential also causes some of the compressed gas from the discharge to leak through the gap that exists between the balance piston outer diameter and the balance piston seal, which is recirculated through the compressor stages, thus increasing the compressor power consumption thereby reducing system efficiency. In other embodiments, the balance piston may be disposed on the compressor shaft at alternate locations.
The thrust bearing and balance piston are oftentimes quite large and therefore occupy a large portion of the axial length of the shaft. Consequently, there is reduced axial space for additional impellers which could otherwise increase the compression capability of the unit. Also, with added mass elements along the shaft, shaft rotordynamics becomes increasingly complicated and the shaft may not be able to operate in a stable way at the required speed levels.
What is needed, therefore, is a system and method of counterbalancing thrust forces generated by centrifugal impellers of a compressor rotor, to reduce the size and weight and at the same time overcomes the disadvantages of the prior systems described above.